This invention relates to semiconductor memory devices, and more particularly to an improved sense amplifier for an N-channel MOS memory device of the type employing one-transistor memory cells.
MOS random access memory (RAM) devices, widely used in the manufacture of digital equipment such as minicomputers, continue to offer increased speed capabilities and cost advantages. The cost per bit of storage using MOS RAMs goes down as the number of bits or memory cells per package goes up. Successively larger RAMs have been standards in the industry. A RAM containing 4096 bits, for example, is shown in U.S. Pat. NO. 3,940,747 issued Feb. 24, 1976 to Kuo and Kitagawa, assigned to Texas Instruments, while 16,384 bit or "16K" RAMs are described in U.S. Pat. Nos. 4,050,061 and 4,081,701, assigned to Texas Instruments, and in articles in Electronics, Feb. 19, 1976, pp. 116-121, and May 13, 1976, pp. 81-86.
As the number of bits in a semiconductor chip is increased, the cell size decreases, and the magnitude of the storage capacitor in each cell of necessity decreases. Also, the number of cells on a digit line in the array of cells increases, so the capacitance of this line increases, and the ratio of the storage capacitance to that of the digit line decreases. These factors increase the difficulty in sensing the data signal which exists on a digit line. A full logic level difference between a "1" and a "0" in one of these devices may be perhaps 5 volts; however, the difference in voltage between a "1" and a "0" for the data coupled to a column line and reaching the sense amplifier in the center of the memory array from the selected one-transistor cell and dummy cell may be only fifty millivolts or less. Various circuits for sensing these low-level signals have been used. Memory cell layouts with sense amplifiers in the center of each column line are shown in U.S. Pat. No. 3,940,747, and the Electronics articles mentioned above, and in U.S. Pat. No. 3,838,404 to Heeren, as well as in Electronics, Sept. 13, 1973, Vol. 46, No. 19, pp. 116-121, and IEEE Journal of Solid State Circuits, Oct. 1972, p. 336, by Stein et al. One of the most widely used sense amplifiers for MOS RAMs at present is the dynamic type used in the 16K and 64K devices manufactured by several companies as part numbers 4027 and 4116. However, these circuits require additional clock voltages for the series transistors between the sense nodes and the bistable transistors and also are of complicated layout.
It is a principal object of this invention to provide an improved sense amplifier for an MOS RAM, and in particular a circuit which is of high speed operation as well as high sensitivity, and which is of small size in layout and simplified in construction.